A laser marking device which employs laser beams as a light source and liquid-crystal elements as printing pattern masks has already been proposed in Japanese Patent Application Laid-open No.60-174671.
In laser marking devices of this kind, the incident face of a liquid-crystal mask is irradiated via an irradiating optical system with laser beams which have been generated by a laser light source. Laser beams which permeate the liquid-crystal mask are emitted from the face opposite to the incident face.
For this reason, it is sought to prevent the optical system comprising the incident and emitting optical systems from spreading spatially to the right and left. This is achieved either by locating the emitting optical system, which guides emitted laser beams on to the object which is to be marked, on the opposite side of the liquid-crystal mask to the incident optical system, or by employing a plurality of optical elements designed to deflect the light emitted from the liquid-crystal mask to the side on which the incident optical system is located.
Whichever of these methods was adopted, the optical system comprising the incident and emitting optical systems required a great deal of space in both the horizontal and vertical directions.
In particular, the method which employs a plurality of optical elements in order to deflect the light emitted from the liquid-crystal mask to the side on which the incident optical system is located requires mirrors, prisms and numerous other optical elements, with the result that it is costly.
Moreover, in order to prevent uneven marking it is necessary to ensure that the liquid-crystal mask is not marred by unevenness of temperature among the pixels of the liquid-crystal screen. In view of the fact that it takes longer to alter the liquid-crystal display if the temperature is too low, and the contrast deteriorates if it is too high, it is clear that there is an optimal temperature for the liquid-crystal mask. Consequently, the liquid crystal mask must be cooled so that the incident face is more even in temperature than the rear face, and so as to ensure that the optimal temperature is maintained.
In the case of conventional laser marking devices it has not been possible to locate a means of cooling in direct contact with the liquid-crystal mask face because they are configured in such a manner that the laser beam is incident on one face of the liquid-crystal mask, permeates it, and is emitted from the other face.
It has thus only been possible to cool it indirectly with a cooling fan or similar device using air as a medium. This method is problematic not only on account of the space required for the cooling device, but because of a lack of uniformity in cooling efficiency.
It is a first object of the present invention, which has been designed with these circumstances in mind, to save space for the optical system comprising the incident and emitting optical systems, and to lower costs by reducing the number of optical elements.
It is a second object of the present to make it possible for a means of cooling to be brought into direct contact with the liquid-crystal element, thus improving the efficiency and uniformity of cooling.